In wireless communications, the term handoff generally refers to the process of transferring an ongoing voice call or data session (collectively, “a call”) from one wireless communication network channel to another wireless communication network channel. The term handoff may also refer to the process of adding or removing additional communication channels to the communication path of a wireless communication device (WCD) communicating on a wireless network. One situation in which a handoff might occur is when a WCD moves away from a first radio-frequency (RF) coverage area source and enters a second RF coverage area. The call may be transferred to the second RF coverage area in order to avoid call termination when the WCD moves outside the first coverage area. As another example, when another channel interferes with the communication channel used by the WCD, the call may be transferred to a different channel in the same coverage area or to a different channel in another coverage area in order to avoid the interference.
Handoffs may also occur when a WCD call moves between an umbrella coverage area and a micro-coverage area, or vice-versa, such as when a call transfers between a typical telecommunications cellular antenna coverage area and a femtocell. A femtocell, also known as an Access Point Base Station, may be a small cellular base station, typically designed for use in residential or small business environments. The femtocell may connect to a service provider's network via broadband (such as DSL or cable) and it typically allows service providers to extend service coverage indoors, especially where access to the umbrella coverage area would otherwise be limited or unavailable. The femtocell incorporates the functionality of a typical base station but extends it to allow a simpler, self-contained deployment. The femtocell concept is applicable to all wireless communication standards, including, but not limited to: time division multiple access (TDMA) systems, global system for mobile communications (GSM) systems, Worldwide Interoperability for Microwave Access (WiMAX or IEEE 802.16) systems, code division multiple access (CDMA or CDMA2000) systems, or time division-synchronous code division multiple access (TD-SCDMA) systems.
Handoffs may generally be categorized as either “soft” or “hard.” A soft handoff is a handoff in which a channel in a first coverage area is retained and used in parallel, at least temporarily, with a channel in a second coverage area. In a soft handoff, the connection to the second coverage area is established before the connection to the first coverage area is broken. The interval during which the two connections are used in parallel may be brief or substantial. A soft handoff may also involve using connections in more than just two coverage areas, for example, one WCD may maintain connections to three, four, or more coverage areas at the same time.
One advantage of soft handoffs is that the connection to the first coverage area is broken only when a reliable connection to the second coverage has been established. Therefore, the chances that the call will be terminated abnormally due to a failed handoff are lower. By adjusting the duration of soft handoffs and the size of the areas in which they occur, network engineers can balance the benefit of extra call reliability against the price of reduced call capacity due to the simultaneous use of multiple channels. Thus, there is a benefit to testing how soft handoffs may occur between coverage areas.
A hard handoff is a handoff in which a channel in a second coverage area is engaged only when the channel in a first coverage area is released. Thus the connection to the first coverage area is broken before the connection to the second coverage area is made. Hard handoffs are intended to be instantaneous in order to minimize the disruption to the call. In general, a hard handoff is usually not perceptible by the user.
An advantage of the hard handoff is that, at any moment in time, a given call uses only one channel. A disadvantage is that if a handoff fails, the call may be temporarily disrupted or even terminated. Just as with soft handoffs, network engineers must weigh and balance the benefits and disadvantages of hard handoffs by, among other things, analyzing the timing, software parameters, and signal parameters of hard handoffs. Thus, there is a benefit to testing how hard handoffs may occur between coverage areas.